1. Field Of The Invention
This invention relates to power conversion and more particularly to an AC--AC series-resonant conversion system with only 12 thyristors.
2. Background Of The Invention
AC--AC series resonant conversion system have been developed because of the many potential applications, such as AC motor drives including the processes of the braking and reversing of the direction of rotation. Another possible application is the asynchronous coupling of two polyphase AC systems, such as interfacing a windmill generator and a power grid, in which a coupling element is required in the form of a power interface. The AC--AC series-resonant power converter shows good performance due to the intrinsic characteristics.
The series-resonant converter employs a modulated train of power pulses to generate a multiphase bipolar waveform. The high resolution obtained for the control and distribution of electric energy paves the way for this process of wave shaping. The fast response has its roots in the high internal frequency and the applied philosophy of control which avoids the interposition of low frequency filters in its signal processing system. A complex process of excitation of the double excited LC-circuit makes it possible to operate the power converter for both directions of the power flow. The natural limitation for the conversion ratio of the conventional series-resonant converter to be one, can be overcome. The developed converter system is studied with respect to the coupling of a three-phase AC voltage source and a three-phase AC load. But the electric energy can also be derived from a two terminal DC source of supply.
The use of a series-resonant circuit for power transfer and control, involves natural current commutation of the thyristors. Improved reliability results from the moderate stresses arising during the switching of semiconductor devices under zero current conditions in the resonant circuit.
The resonant circuit is considered to be a high-frequency alternating-current link between two sets of switching matrices at the input and output terminals of the AC--AC inverter. This high-frequency link allows the elimination of all low-frequency filters and transformers, which are a requirement by conventional AC--AC converters because of the interposition of a low-frequency direct-voltage or direct-current link between two sets of switching matrices. The generation of high-frequency components in the source and load currents can be reduced to relatively small quantities by the application of high-frequency filters of moderate size.
Direct operation by an AC-current link requires power switches to have bidirectional blocking and conducting capabilities. Bidirectional current capability is also mandatory for bipolar source and load currents. The more conventional series-resonant AC--AC converter applies, therefore, bipolar switches in arrangements of anti-parallel thyristors with a total of 24 individual thyristors.
Therefore, it is an object of the present invention to provide an improved series-resonant AC--AC converter with only 12 thyristor switches.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed as being merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention with in the scope of the invention. Accordingly other objects in a full under standing of the invention may be had by referring to the summary of the invention, the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.